Enhance notification centre with better event handling

[Revolyssup]

Signed-off-by: ashish ashishjaitiwari15112000@gmail.com

Description

This PR

1. Ensures that, in case multiple clients are connected to single Meshery instance then all events on that Meshery instance are forwarded to all connected clients.
2. Removes a redundant global channel.

Notes for Reviewers This is a breaking change and appropriate changes would be needed in adapters.

Signed commits

• [x] Yes, I signed my commits.

[Revolyssup]

@MUzairS15 We will start our discussion from here.